Degradable plastics are used as a way to decrease the environmental impact of these materials when they become waste. However, they can reach natural ecosystems due to littering and bad management. This research assesses the performance of oxodegradable and compostable plastics on marine environments through a respirometric lab test. Probes of the plastics, with and without previous simulated weathering, were put in contact for 48 days with a marine inoculum, in a system that guarantees continuous aeration and capture of the produced CO2. After the test, the samples were also assessed in terms of their loss of mechanical properties. The compostable plastic exhibited the higher degree of mineralization (10%), while there was no difference between the polyolefins (2.06% - 2.78%), with or without presence of pro-oxidants or previous abiotic degradation. On the other hand, exposition to UV light promoted a higher loss of elongation at break in the oxodegradables plastic (>68%). The results show that the studied plastics achieve very low biodegradation rates while presenting a higher rate of loss of physical integrity. This combination of phenomena could lead to their fragmentation before significant biodegradation can occur. The risk of microplastics formation must be prevented by avoiding the presence of the materials in marine environments, even if they have shown suitability for some waste management scenarios.